Abstract
The security of prepare-and-measure satellite-based quantum key distribution (QKD), under restricted eavesdropping scenarios, is addressed. We particularly consider cases where the eavesdropper, Eve, has limited access to the transmitted signal by Alice and/or Bob’s receiver station. This restriction is modeled by lossy channels between relevant parties, where the transmissivity of such channels can, in principle, be bounded by monitoring techniques. An artifact of such lossy channels is the possibility of having bypass channels, those that are not accessible to Eve but that may not necessarily be characterized by the users either. This creates interesting unexplored scenarios for analyzing QKD security. In this paper, we obtain generic bounds on the key rate in the presence of bypass channels and apply them to continuous-variable QKD protocols with Gaussian encoding with direct and reverse reconciliation. We find regimes of operation in which the above restrictions on Eve can considerably improve system performance. We also develop customized bounds for several protocols in the BB84 family and show that, in certain regimes, even the simple protocol of BB84 with weak coherent pulses is able to offer positive key rates at high channel losses, which would otherwise be impossible under an unrestricted Eve. In this case, the limitation on Eve would allow Alice to send signals with larger intensities than the optimal value under an ideal Eve, which effectively reduces the effective channel loss. In all these cases, the part of the transmitted signal that does not reach Eve can play a nontrivial role in specifying the achievable key rate. Our work opens up new security frameworks for spaceborne quantum communications systems.
- Received 20 December 2022
- Revised 28 April 2023
- Accepted 25 September 2023
DOI:https://doi.org/10.1103/PRXQuantum.4.040320
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
Quantum communications allows individual users to securely exchange data with minimal assumptions on the computational power of potential eavesdroppers. Being based on fundamental physical systems, such as photons, quantum secure protocols often suffer from loss in the transmission channel and, hence, are limited in the distance scales they can cover. Fortunately, progress with satellite systems can help with this issue. We look at the security assumptions that are relevant where quantum protocols are implemented via spaceborne systems and speculate how their performance can be improved under realistic conditions.
One of the key assumptions in quantum secure protocols is that the entire channel is assumed to be controlled by an eavesdropper. The protocol is then designed in such a way that, on the basis of the transmitted signals and the measurement results obtained, the two legitimate users can bound the amount of information that may have leaked to the eavesdropper. In satellite-based quantum communications, one may argue that this main assumption is a bit too conservative, and it might be of practical interest to loosen it a bit and see how system performance is improved. This is the key motivation for our work, in which some restrictions are imposed on potential eavesdroppers. The restrictions we assume have practical relevance in the satellite setting and result in scenarios that are less explored in the field. We provide new generic and customized bounds for the security of the system in these new scenarios. In particular, some protocols that may not be space operational under unrestricted assumptions will become viable solutions in our framework, which makes the uptake of the technology more feasible in the near future.
